It is well known that many metallic components or parts need a final surface treatment. Such a surface treatment increases functionality and the lifetime of the part by improving one or more properties of the part, such as heat resistance, corrosion protection, wear resistance, hardness, electrical conductivity, lubricity or by simply increasing the cosmetic value.
One example of a part that is typically surface treated is the head of aluminum pistons used in combustion engines. (As used herein an aluminum component is a component at least partially comprised of aluminum, including aluminum alloys.) Such piston heads are in contact with the combustion zone, and thus exposed to relatively hot gases. The aluminum is subjected to high internal stresses, which may result in deformation or changes in the metallurgical structure, and may negatively influence the functionality and lifetime of the parts. It is well known that formation of an anodic oxide coating (anodizing) reduces the risk of aluminum pistons performing unsatisfactorily. Thus, many aluminum piston heads are anodized.
There is a drawback to anodizing piston heads. Conventional anodizing with direct current or voltage, increases the surface roughness of the initial aluminum surface by applying an anodic coating. The increase in surface roughness can be as high as 400%, depending on the aluminum alloy and process conditions. The amount of VOC (Volatile Organic Compounds) in the exhaust of a combustion engine is correlated with the surface finish of the anodized aluminum piston: higher surface roughness reduces the efficiency of the combustion, because a greater proportion of organic compounds can be trapped in micro cavities more easily. Therefore, a smooth surface is required, which may not always be provided by anodization.
A typical prior art power supply for the conversion of metallic aluminum into a ceramic coating (aluminum oxide or alumna) provides direct current, normally between 3 and 4 A/dm2. Typically, a film thickness of 20 to 25 microns is reached after 30 to 40 minutes.
Convention anodizing includes subjecting the aluminum to an acid electrolyte, typically composed of sulfuric acid or electrolyte mixed with sulfuric and oxalic acid. The anodizing process is generally performed in electrolytes containing 12 to 15% v/v sulfuric acid at relatively low process temperature, such as from -5 to +45 degrees C. Higher concentrations and temperature usually decrease the formation rate significantly. Also, the formation voltage decreases with higher temperature, which adversely affects the compactness and the technical properties of the oxide film.
Performing anodizing process at (relatively) low temperature and fairly high current density increases the compactness and technical quality of the coating performance (high hardness and wear resistance). The anodization produces a significant amount of heat. Some heat is the result of the exothermic nature of the anodizing of aluminum. However, the majority of the heat is generated by the resistance of the aluminum towards anodizing. Typically, the reaction polarization is high, such as from 15-30 volts, depending upon the composition of the alloying elements and the process conditions. Given typical current densities, from 80% to 95% of the total heat production will be resistive heat.
The electrolyte is acidic, and thus chemically dissolves the aluminum oxide. Thus, the net formation of the coating (aluminum oxide) depends on the balance between electrolytic conversion of aluminum into aluminum oxide and chemical dissolution of the formed aluminum oxide.
The rate of chemical dissolution increases with heat. Thus, the total production of heat is a significant factor influencing this balance and helps determines the final quality of the anodic coating. Heat should be dispersed form areas of production toward the bulk solution at a rate that prevents excess heating of the electrolytic near the aluminum part. If the balance between formation and dissolution is not properly struck, and dissolution is favored, the oxide layer may develop holes, exposing the alloy to the electrolyte. This often happens in prior art anodization methods and is known as a "burning phenomena".
Heat produced at the aluminum surface is dispersed by air agitation or mechanically stirring of the electrolyte in which the oxidation of aluminum is taking place, in the prior art, to help reach the desired balance.
Another way of dispersing the heat is by spraying the electrolyte toward the aluminum surface (U.S. Pat. No. 5,534,126 and U.S. Pat. No. 5,032,244). The electrolyte is sprayed toward the aluminum surface at an angle of 90 degrees, moving heat toward the areas of production, and then symmetrically dispersed away from the aluminum surface.
Another way to disperse heat is to pump the electrolyte over the aluminum substrate (U.S. Pat. No. 5,173,161). The electrolyte is moved parallel to the aluminum surface, moving heat from the lower part of the aluminum substrate over the entire surface before it is finally dispersed away from the aluminum surface.
A steady state transport mechanism in electrochemical analysis (not anodization) techniques based on wall jet processes can be achieved by either rotating the working electrode, or by directing the flow toward a stationary electrode, at an angle of between 60 and 70 degrees. By angling the jet stream of the reaction medium to 60-70 degrees where steady state conditions are obligatory, electrochemical analysis can be made. Steady state conditions in a jet stream orthogonal to the working electrode is less suitable for wall jet electrochemical analysis. The inventor is not aware of this information having been applied to an electrolytic process.
The driving force of the charge-transfer reaction taking place at the substrate surface in the process described in U.S. Pat. Nos. 5,032,244, 5,534,126 and 5,173,161, was direct current. The reaction medium was a solution of sulfuric acid or a combination of sulfuric and oxalic acid in U.S. Pat. No. 5,032,244. The electrolyte formulation was 180 g/l sulfuric acid and the process temperature was +5 degrees C. A current density of 50 A/dm2 produced a coating with a thickness of 65 microns in 3 minutes. The microhardness of the obtained coating was between 200 and 300 HV.
A second process included the addition of 10 g/l oxalic acid at the same current density. A coating having a thickness of more than 60 microns and having a microhardness greater than 400 HV was obtained in 5 minutes.
After anodizing, the aluminum parts are typically rinsed and dried. Both anodizing, rinsing and drying is made in the same process chamber in all three U.S. patents mentioned above. Some chambers have at least two aluminum parts (see U.S. Pat. Nos. 5,534,126 or 5,173,161). Others have a single part in each chamber (see U.S. Pat. No. 5,032,244).
Conventional batch anodizing has used square wave alternation of current or potential. This allows anodizing to be performed at higher current densities compared to anodizing with direct current. The pulse anodizing is characterized by a periodically alternation between a period with high current or voltage, during with the film is formed, and a period with low current or voltage, during which heat is dispersed (U.S. Pat. No. 3,857,766). This technique utilizes the "recovery effect", after a period of high formation rate (a pulse period), heat is allowed to disperse during the following period with low formation rate (a pause period) and defects in the coating are repaired before the current increases during the next pulse. The relative durations of the higher magnitude and lower magnitude currents determine the relative amount of oxide formation and heat dispersion. One such type of simple pulse pattern may be found in U.S. Pat. No. 3,857,766 or Anodic Oxidation of Al. Utilizing Current Recovery Effect, Yokohama, et al. Plating and Surface Finishing, 1982, 69 No. 7, 62-65.
U.S. Pat. No. 3,983,014, entitled Anodizing Means And Techniques, issued Sep. 28, 1976 to Newman et al., discloses another type of pulse pattern. The pulse pattern described in Newman has a high positive current portion, followed by a zero current portion, followed by a low negative current portion, followed again by a zero current portion. Each of the pulse portions represent one quarter of the cycle. Thus, the current has a high positive value during the first quarter of the cycle. No current is provided during the next quarter of the cycle. The current has a low negative value during the third quarter cycle. Zero current is provided during the final quarter of the cycle.
Another prior art pulse pattern is described in U.S. Pat. No. 4,517,059, issued May 14, 1985, to Loch et al. Loch discloses a pulse pattern that is a square wave alternating between a relatively high positive current and a relatively low negative current. The durations of the positive and negative portions of the pulses are controlled used in an attempt to control the anodizing process.
U.S. Pat. No. 4,414,077, issued Nov. 8, 1983, to Yoshida et al. describes a train of pulses superimposed on a dc current. The pulses are of a plurality opposite to that of the dc current.
Other prior art methods use a sinusoidal voltage wave, or portions thereof, applied to the voltage buses used for generating the anodizing currents (i.e. potentiostatic pulses). However, such prior art systems do not utilize current pulses for controlling the anodizing process. Examples of such prior art systems may be found in U.S. Pat. No. 4,152,221, entitled Anodizing Method, issued May 1, 1979, to Schaedel; U.S. Pat. No. 4,046,649, entitled Forward-Reverse Pulse Cycling Pulse Anodizing And Electroplating Process issued Sep. 6, 1977, to Elco et al; and U.S. Pat. No. 3,975,254, entitled Forward-Reverse Pulse Cycling Anodizing And Electroplating Process Power Supply, issued Aug. 17, 1976, to Elco et al.
Each of the aforementioned prior art methods, while utilizing a pulse of some sort, does not provide adequate hardness and thickness while maintaining a low reject rate. Moreover, such prior art systems are relatively slow and take a relatively long period of time to complete the anodizing process.
The time of each period is typically ranges from 1 to 100 seconds in the prior art, depending on the aluminum substrate. The prior art does not describe a correlation between a pulse pattern (pulse current, pulse duration, pause current and pause duration) and the result of the anodizing process. (See Yokogama, above). Thus, the optimal pulse conditions have been determined by trial and error. The coating quality of pulse anodized aluminum is generally superior to anodic coatings produce with direct current according to the prior art (Surface Treatment With Pulse Current, Dr. Jean Rasmussen, December 1994.)
An anodizing method and apparatus that reduces processing time with high formation potentials and minimal handling to obtain coatings of desirable quality and consistency is desirable. The process and apparatus will preferably lessen production costs and have a closed loop process design that reduces the impact of the electrolyte on internal and external environments. The process will preferably remove heat from near the component being anodized.